The present invention relates to an electric power steering apparatus.
FIG. 4 is a block diagram of an electric power steering apparatus 100 described in Japanese Patent No. 3154665. In the drawing, controller 101 controls a steering auxiliary motor M based on a vehicle speed output from a vehicle speed sensor and a steering torque output from a torque sensor.
In the drawing, reference numeral 102 denotes an H bridge circuit. The H bridge circuit 102 is driven by an FET driving circuit 103. The FET driving circuit 103 is subjected to PWM (pulse-width modulation) control by the controller 101 based on a PWM signal.
The controller 101 obtains a difference between a current command value associated with the vehicle speed and the steering torque, and a current actually flowing through the steering auxiliary motor M. The controller then performs PWM control with respect to the FET driving circuit 103 based on this difference.
The current actually flowing through the steering auxiliary motor M is obtained by detecting a voltage across a motor current detection resistance 104.
The voltage across the motor current detection resistance 104 is supplied to a sample-hold circuit 105 and held as a voltage across a capacitor C1 in the sample-hold circuit 105.
The voltage held in the capacitor C1 is input to an operational amplifier 106 as a voltage across the input terminals of the operational amplifier 106. The voltage is amplified in the operational amplifier 106.
An output from the operational amplifier 106 is supplied to a base of a PNP transistor Q10 to turn the transistor Q10 on, which causes a collector current to flow. The collector current flows through a collector resistance R14 and is converted into a voltage, which is read into the controller 101. Since the collector current in the PNP transistor Q10 is associated with a magnitude of the voltage across the motor current detection resistance 104, the controller 101 can obtain a magnitude of the current actually flowing through the steering auxiliary motor M based on the magnitude of the collector current.
However, this conventional electric power steering apparatus 100 has the following disadvantage. Since the operational amplifier 106 generally has an offset voltage, when a current flowing through the motor current detection resistance 104 is small, the current cannot be detected because of the influence of the offset voltage. Therefore, there is a problem in that accurate assist control cannot be executed by the electric power steering apparatus 100.
Japanese Published Patent Application No. 1992-251596 discloses another electric power steering apparatus, in which an influence of an offset voltage is eliminated. FIG. 5 illustrates the device described in that document. A voltage across a shunt resistance Rs is detected by a differential circuit 51. A predetermined voltage Vr is applied to a positive input terminal of the differential circuit 51. The voltage detected by the differential circuit 51 is supplied to a negative input terminal of a comparator 53. The comparator 53 executes half-wave rectification. A connection point between the shunt resistance Rs and a battery 48 is connected to a positive input terminal of a differential circuit 52. The predetermined voltage Vr is also applied to the positive input terminal of this differential circuit 52. The differential circuit 52 compensates for an offset voltage generated by the differential circuit 51 and the comparator 53.
However, in the device described in Japanese Published Patent Application No. 1992-251596, which is thus configured to compensate for the offset voltage, there is a problem in that the circuit configuration is complicated and selection of its respective elements is troublesome. A device that can easily and accurately execute assist control by enabling compensation for the offset voltage but with a simple structure has thus been demanded.
Another known electric power steering apparatus is described in Japanese Published Patent Application No. 2002-46630.
In the electric power steering apparatus disclosed in that document, a current value flowing through a motor current detection resistance is held in a peak hold circuit and then fetched into a controller.
FIG. 6 shows the peak hold circuit 60 described in Japanese Published Patent Application No. 2002-46630. This peak hold circuit 60 includes a first peak hold circuit 61 and a second peak hold circuit 62. Operation of the first peak hold circuit 61 will now be explained. When a motor current flows through a motor current detection resistance 56, a potential at a non-inverting input terminal of a comparator 61a (a potential at a point U) is increased. When this potential at the non-inverting input terminal of the comparator 61a is higher than a potential at an inverting input terminal of the comparator 61a, the comparator 61a is turned off and a diode 61c is turned on. As a result, a current flows through a capacitor 61f via a resistance 61g and the diode 61c, and the capacitor 61f is charged. Based on this charge, when the potential at the non-inverting input terminal of the comparator 61a becomes higher than the potential at the non-inverting input terminal of the comparator 61a, the comparator 61a is turned on, the diode 61c is turned off, charging of the capacitor 61f is stopped, and discharging of the capacitor 61f is carried out via resistances 61d and 61e. An operational amplifier 61b converts a potential VP1 at a point P1 into a low impedance and outputs this impedance to a channel CHI of an ADC 43. Respective values of elements in the first peak hold circuit 61, e.g., resistance values R1 and R2 or a capacitance of the capacitor 61f are selected in such a manner that a maximum value of an analog voltage VP1 (the analog voltage VP1 when a motor current has a maximum value that can be taken under control) becomes equal to a maximum value of a voltage that can be subjected to analog/digital conversion by the ADC 43.
However, the invention described in Japanese Published Patent Application No. 2002-46630 has the following problem. FIG. 7(A) shows a waveform of a PWM signal. FIG. 7(B) depicts a voltage generated in the motor current detection resistance 56. Furthermore, as shown in FIG. 7(B), a time constant that is determined based on the capacitance of the capacitor 61f and the resistance values R1 and R2 of the resistances 61d and 61e is set with respect to the capacitor 61f so that the same voltage as the voltage generated in the motor current detection resistance 56 can be produced. However, when the time constant is too large, discharge of the voltage in the capacitor 61f takes a long time, and a voltage generated in the motor current detection resistance 56 in synchronization with the next PWM signal is thereby added before the voltage in the capacitor 61f is completely discharged. Therefore, the voltage in the capacitor 61f becomes a voltage higher than the voltage currently generated in the current detection resistance 56, thus leading to a problem in that the motor current cannot be accurately detected.
When the time constant is too small, the capacitor 61f cannot hold the voltage for a fixed time and can readily discharge the voltage. As a result, the voltage in the capacitor 61f is discharged before the controller reliably fetches the voltage in the capacitor 61f, thereby leading to a problem in that the controller cannot execute accurate assist control.
That is, in the invention disclosed in Japanese Published Patent Application No. 2002-46630, when performing accurate assist control, there occurs a problem that adjustment of the time constant determined based on the capacitor 61f and the resistance values R1 and R2 of the discharge resistances 61d and 61e is complicated.
Therefore, in view of the problems in the conventional technology, it is an object of the present invention to provide an electric power steering apparatus that has a simple structure and which enables highly accurate detection of a current value actually flowing through a steering motor to execute accurate assist control.